Gary,

I do ENIG in Chippewa Falls, and Thickness is the only major specification.  The planarity is not an issue.  As far as thickness goes typical spec is 3-8 microinches of gold and 150-250 microinches of nickel.  The gold simply needs to be thick enough to get a good oxidation free surface while the nickel thickness is more critical.  While very thick nickel is costly both due to material expense and greatly reduced thoughput, thin nickel can be unreliable for several reasons, primarily due to the migration of copper through the nickel and up to the gold surface.  The has been some debate over the reality of this occurrence - but I have seen it for myself and have elemental SEM results that support it.  This usually occurs when you drop below 100 microinches of nickel.  

One other specification that I have seen that is not as common, is P-Weight in the nickel.  Problem with that is that most fab shops don't have the capability to accurately analyze P-Weight.  The general consensus is that you want to keep the phosphorus between 6 and 10 % in the nickel deposit however this is typical of nearly all of the ENIG processes available so it's not as useful a spec as the thickness.  

Hope this serves as some help to you.

Regards

Joe Aronson
Wet Process Engineer
Allied Signal Electronic Materials

>>> Gary Crowell <[log in to unmask]> 09/02/99 10:38AM >>>
We're doing our first board using high pincount BGA's, and I understand
that we should be an immersion gold finish to get the flatness needed
for BGA assembly.  My question is, how do I specify this type of finish
to the fab house?  I assume thickness is one of the factors; what is
typical and what are the considerations in thickness spec?  What else
needs to be specified?

The board in question is 9x14", 18 layers, a prototype build on a board
that will never see high volumes (about 100 units max lifetime
production).  Environment is laboratory, internal company use only.

TIA

Gary Crowell
Micron Technology

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